Quoting Jeff Moyer (2013-11-07 11:14:02) > Chris Mason <chris.mason@xxxxxxxxxxxx> writes: > > >> Well, we have control over dm and md, so I'm not worried about that. > >> For the storage vendors, we'll have to see about influencing the > >> standards bodies. > >> > >> The way I see it, there are 3 pieces of information that are required: > >> 1) minimum size that is atomic (likely the physical block size, but > >> maybe the logical block size?) > >> 2) maximum size that is atomic (multiple of minimum size) > >> 3) whether or not discontiguous ranges are supported > >> > >> Did I miss anything? > > > > It'll vary from vendor to vendor. A discontig range of two 512KB areas > > is different from 256 distcontig 4KB areas. > > Sure. > > > And it's completely dependent on filesystem fragmentation. So, a given > > IO might pass for one file and fail for the next. > > Worse, it could pass for one region of a file and fail for a different > region of the same file. > > I guess you could export the most conservative estimate, based on > completely non-contiguous smallest sized segments. Things larger may > work, but they may not. Perhaps this would be too limiting, I don't > know. Depends on the workload. For mysql, they really only need ~16KB in the default config. I'd rather not restrict things to that one case, but it's pretty easy to satisfy. > > > In a DM/MD configuration, an atomic IO inside a single stripe on raid0 > > could succeed while it will fail if it spans two stripes to two > > different devices. > > I'd say that if you are spanning multiple devices, you don't support > O_ATOMIC. You could write a specific dm target that allows it, but I > don't think it's a priority to support it in the way your example does. > > Given that there are applications using your implementation, what did > they determine was a sane way to do things? Only access the block > device? Preallocate files? Fallback to non-atomic writes + fsync? > Something else? Admin opt-in on single drives only. mysql exits with errors if the atomics aren't supported. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
